1. FIELD OF THE INVENTION
This invention relates to an apparatus for detecting the concentration or mixing ratio of pigmented toner particles in the developing powder comprising a mixture or magnetic carrier particles and pigmented toner particles used with electrostatic printers or like.
2. DESCRIPTION OF THE PRIOR ART
In an apparatus wherein an electrostatic latent image is developed by the developing powder comprising a mixture of magnetic carrier particles and pigmented toner particles, the toner particles are consumed while the magnetic carrier particles remain without being consumed and are reused in the course of developing processes. After repeated use of the developing powder, the concentration of the toner particles in the developing powder gradually decreases. In order to maintain a high quality of development, the concentration of the toner particles is required to be maintained constant or in a predetermined range, which in turn requires the detection of the concentration of the toner particles. Toner concentration detecting apparatuses operated in response to variations in permeability of the developing powder were suggested. Such apparatuses include U.S. Pat. No. 3,572,551 entitled "Apparatus for Monitoring and Controlling the Concentration of Toner in a Developer Mix" invented by Henderson et al., patented Mar. 30, 1971 on application Ser. No. 811,132 filed Mar. 27, 1969, and assigned to RCA Corporation. The present inventor suggested, on the other hand, improved toner concentration detecting apparatuses in U.S. appli. Ser. No. 783,554 entitled "Toner Concentration Detecting Apparatus" invented by Isamu Terashima and filed Apr. 1, 1977, now U.S. Pat. No. 4,131,081, and in U.S. appl. Ser. No. 841,737 entitled "Toner Concentration Detecting Apparatus" invented by Isamu Terashima, filed Oct. 13, 1977, now U.S. Pat. No. 4,147,127. These inventions have the disadvantage that the flow (density) of the developing powder acting on the detecting coil is unstable or likely to be affected by an external magnetic circuit. The U.S. Pat. No. 3,802,381 entitled "Apparatus for Measuring Concentration Ratio of a Mixture of Materials" invented by Roger M. O'Neill et al., patented Apr. 9, 1974 on application Ser. No. 140,573 filed May 5, 1971 and assigned to Continental Can Company, Inc. suggests, in contrast, that the detecting coil be inserted into a detecting cylinder. Such an apparatus also poses the problem of the flow of the developing powder being disturbed by the detecting coil, thus making difficult the measurement of high accuracy. The prior art other than those cited above includes U.S. Pat. No. 3,999, 687.